In modern cement manufacturing plants, the raw materials are typically heated by heat exchange with exhaust gases from the kiln and/or the calciner. This takes place in a so-called suspension preheater, typically a tower made up of a number of cyclones connected in series. Calcination is typically carried out in a duct section at the bottom of the tower where the temperature and retention time are sufficient for calcination, i.e. the decomposition of limestone, to take place, subject to release of carbon dioxide. The calciner may, viewed in the direction of movement of the material, be installed immediately ahead of the rotary kiln, such as in an ILC installation and/or in the duct with hot air from the cooler, such as in an SLC installation. The calciner is fed with so-called secondary fuel, and combustion hereof is effected by means of oxygen which is in surplus from the kiln or contained in the hot air from the cooler. Typically, a small amount of kiln gases are removed via a bypass to prevent accumulation of certain components in the process. The calcined material is then directed to the kiln, in most cases a rotary kiln, in which the subsequent heating is done in counterflow to exhaust gases from a primary burner. After being burned in the rotary kiln, the clinker is directed to a clinker cooler in which the clinker is cooled to a level around 100 degrees Celcius by means of atmospheric air.
The energy consumed in connection with the cement production process at a modern plant, as described above, is approximately 3 MJ/kg clinker. By comparison, the theoretical minimum energy consumption is approximately 1.7 MJ/kg. The difference between the actual and the theoretical energy consumption is primarily due to loss from hot surplus air from the clinker cooler and heat loss from the exhaust gases which are diverted from the preheater. The specific emission of carbon dioxide is approximately 1 kg CO2/kg clinker, of which approximately one half relates to the limestone contained in the cement raw meal, whereas the other half relates specifically to the fuel. It is estimated that the contribution made by cement manufacturing processes to the global human-caused emissions of the greenhouse gas carbon dioxide is approximately 5 percent, and, therefore, it would be highly desirable to reduce this level.
Some plants utilize a portion of the heat content in the surplus air from the cooler and the exhaust gases from the preheater to produce electricity, so-called co-generation. However, the available amount of energy is at a relatively modest level, typically on the order of 5-10 MW heat for a plant with a capacity rating of 5000 t clinker/24-h, corresponding to 3-6% of the rated thermal input. Also, the heat available will be of low grade, i.e. its efficiency in relation to electricity production will be relatively small, i.e. at a level around 15-20%. Because of these factors, the benefits gained by setting up a co-generation plant will in many cases not be commensurate with the costs involved.